Wood siding panels come in several different styles. Most are made of thin plies of hard or soft wood, laminated together with a specialized glue between the layers. Siding panels are available in vertical-, board- and batten-style panels and horizontal clapboard styles. Most come primed, but should be painted as soon as possible to prevent the absorption of moisture, which can cause swelling and delamination.
Plywood panels come in varying thicknesses, from one-quarter to 1 inch. Most plywood can be used for exterior projects with a coat of paint. Marine-grade plywood is laminated with a special water-resistant glue specifically designed to repel water. Green pressure-treated plywood is designed to be used in all exposures without additional sealers, but it can be painted if you so choose.
OSB sheathing is a special type of moisture-resistant chipboard that is used as wall and roof-deck sheathing in place of plywood. This product has a consistent thickness and is easy to cut and fasten. It can be left exposed temporarily, but it's designed to be covered with siding as an outside layer. Chipboard exposed to the elements more than a few days develops a rough surface, and can swell or crumble.
Hardboard, commonly known by the brand name Masonite, is a hard sheet made of fine wood pulp pressed into a hard sheet that is steam treated for hardness and always pressed with at least one smooth side. Hardboard typically comes in thin one-eighth-to-one-quarter-inch thick sheets that are very flexible. It can also be pressed into siding, with a wood grain appearance that is very durable. Although hardboard is more resistant to moisture than some other wood panels, it should be painted for best results.